The present invention is directed to sheet-feed tractors of the type employed in connection with printers used with computers, word processors, and the like.
The explosion in the use of data and word processing in recent years has resulted in a volume of sales that has caused significant cost competition not only in the end products but also in the components employed. Among these components are the sheet-feed tractors, which are used to drive perforated paper on which automated typing takes place.
An example of a sheet-feed tractor that is so arranged as to permit production at relatively low expense is one produced by Data Motion of Torrington, Connecticut, and described in U.S. patent application Ser. No. 128,590 of Alan F. Seitz, now U.S. Pat. No. 4,315,585, for a Sheet-Feed Tractor with Eccentric Clamping Device. The tractor is made almost exclusively of molded plastic parts and includes two generally parallel frame halves between which is mounted a molded sprocket. A molded drive belt is trained about the sprocket, and tabs on a clamp part are held between the frame parts to hold the molded clamp on the tractor. With the exception of pins used for mounting a molded cover and a spring used in holding it down, this tractor is made entirely of molded plastic, and parts are snap fit or friction fit together.
This type of tractor has advantages of low cost and reliability, but attempts have been made to further reduce the cost associated with tractor manufacture. One attempt is evidenced by U.S. Pat. No. 4,199,091 to Hubbard, which discloses a tractor is which one frame half has flexible rods with latches for holding the other frame half in assembly. The intent is that the tractor be easily disassembled by manual deflection of the rods. Whatever the benefits of this arrangement may be, the clamp parts must be snap fit in such a way as to present considerable resistance to disassembly.
The object of the present invention is therefore a tractor that can be readily assembled and disassembled without the use of tools or excessive exertion. It is a related object of the present invention to achieve this in a tractor that permits variation in the spacing between the support and drive shafts during operation and yet employs a minimum number of parts.